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Melatonin receptors in red deer fetuses (Cervus elaphus)

Red deer (Cervus elaphus) exhibit highly seasonal rhythms in physiology and behaviour. The influence of photoperiod on the timing of these changes begins in utero where the fetus receives photoperiodic information via the diurnal pattern of maternal melatonin secretion. The potential sensitivity of deer fetuses to melatonin was ascertained by mapping specific receptors and characterizing them using 2-[¹²⁵I]iodomelatonin and quantitative autoradiography in vitro. Specific binding occurred from day 31 of gestation onwards (term = 233 days) over the spinal nerves and respiratory system. At later stages of gestation binding occurred over the brain, particularly the brainstem, the pituitary gland, thyroid gland, gastrointestinal tract including the pancreas, metanephros, cochlea of the ear, spinal cord, and spinal and cranial nerves. Binding was abolished in the presence of 10^–7 mol melatonin l^–1 and diminished in the presence of 10^–4 mol GTPS l^–1 (guanosine-5-0-(3-thiotriphosphate)), confirming that binding represented functional G-protein-coupled melatonin receptors. Characterization studies, carried out on fetal lung, revealed that binding was time-dependent, reaching equilibrium at about 3 h at room temperature (22°C), and saturable with a dissociation constant (K_d) of 104 pmol l^–1. This study demonstrates the presence of G-protein-coupled melatonin receptors over a wide range of tissues in red deer fetuses from early in gestation, indicating that in addition to its role in the communication of photoperiodic information to the fetus in this species, melatonin may be involved in fetal growth and development.

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